Fluorescence imaging techniques have been widely used in studying biological samples and the like. To take a fluorescent image of a sample, the sample is illuminated with an excitation light beam which has an frequency band selected for exciting certain fluorophors in the sample. The fluorescent light generated by the excited fluorophors is then collected by light detectors to form an image of the sample.
Based on the type of light source used for generating the excitation light, existing fluorescence imaging devices can be generally divided into laser-based and white light-based devices. A laser-based fluorescence imaging device uses a laser as the light source. There are several disadvantages associated with a laser-based imaging device. A laser generally provides only one or at most a few usable frequencies for fluorescence excitation and therefore does not provide flexibility in the selection of excitation frequency. Moreover, the application of lasers for fluorescence excitation is relatively expensive.
A white light-based fluorescence imaging device uses a light source that generates light over a broad frequency spectrum. The broad-band light (which is typically called white light or polychromatic light) from the light source is filtered to provide excitation light within a narrow band around a selected excitation frequency. The light generated by the white light source is typically transmitted to the sample via an optical fiber bundle. Due to the high numerical aperture of the optical fiber bundle, the excitation beam tends to be highly divergent, resulting in a low illumination intensity on the sample. Another significant disadvantage of such a lighting arrangement is that the imaging system tends to be fairly bulky and less suitable for field applications.
The amount of time required to take a fluorescent image is a critical performance parameter of a fluorescence imaging device. To improve the image acquisition rate, both the efficiency of sample illumination and the efficiency of fluorescent light detection have to be maximized. A high image acquisition rate is especially difficult to achieve when a relatively large area on the sample (typically on the order of one square centimeter) is to be imaged. Some existing laser-based imaging devices focus the input laser beam into a small spot which is scanned across the sample surface to obtain a fluorescent image of the sample. Such a device, for example, is shown in U.S. Pat. No. 5,672,880 to Kain. The disadvantages of an imaging device using a scanning laser beam includes the long scanning time required for acquiring one image and the complexity and costs of the scanning mechanism.